Mechanical and chemical-mechanical planarizing processes (collectively “CMP”) are used in the manufacturing of microelectronic devices for forming a flat surface on semiconductor wafers, field emission displays and many other microelectronic-device substrates and substrate assemblies. FIG. 1 schematically illustrates a CMP machine 10 having a platen 20. The platen 20 supports a planarizing medium 40 that can include a polishing pad 41 having a planarizing surface 42 on which a planarizing liquid 43 is disposed. The polishing pad 41 may be a conventional polishing pad made from a continuous phase matrix material (e.g., polyurethane), or it may be a new generation fixed-abrasive polishing pad made from abrasive particles fixedly dispersed in a suspension medium. The planarizing liquid 43 may be a conventional CMP slurry with abrasive particles and chemicals that remove material from the wafer, or the planarizing liquid may be a planarizing solution without abrasive particles. In most CMP applications, conventional CMP slurries are used on conventional polishing pads, and planarizing solutions without abrasive particles are used on fixed abrasive polishing pads.
The CMP machine 10 can also include an under-pad 25 attached to an upper surface 22 of the platen 20 and the lower surface of the polishing pad 41. A drive assembly 26 rotates the platen 20 (as indicated by arrow A), and/or it reciprocates the platen 20 back and forth (as indicated by arrow B). Because the polishing pad 41 is attached to the under-pad 25, the polishing pad 41 moves with the platen 20.
A wafer carrier 30 is positioned adjacent the polishing pad 41 and has a lower surface 32 to which a substrate 12 may be attached via suction. Alternatively, the substrate 12 may be attached to a resilient pad 34 positioned between the substrate 12 and the lower surface 32. The wafer carrier 30 may be a weighted, free-floating wafer carrier, or an actuator assembly 33 may be attached to the wafer carrier to impart axial and/or rotational motion (as indicated by arrows C and D, respectively).
To planarize the substrate 12 with the CMP machine 10, the wafer carrier 30 presses the substrate 12 face-downward against the polishing pad 41. While the face of the substrate 12 presses against the polishing pad 41, at least one of the platen 20 or the wafer carrier 30 moves relative to the other to move the substrate 12 across the planarizing surface 42. As the face of the substrate 12 moves across the planarizing surface 42, material is continuously removed from the face of the substrate 12.
CMP processes should consistently and accurately produce a uniformly planar surface on the substrate to enable precise fabrication of circuits and photo-patterns. During the fabrication of transistors, contacts, interconnects and other features, many substrates develop large “step heights” that create a highly topographic surface across the substrate. Yet, as the density of integrated circuits increases, it is necessary to have a planar substrate surface at several stages of processing the substrate because non-uniform substrate surfaces significantly increase the difficulty of forming sub-micron features. For example, it is difficult to accurately focus photo-patterns to within tolerances approaching 0.1 μm on non-uniform substrate surfaces because sub-micron photolithographic equipment generally has a very limited depth of field. Thus, CMP processes are often used to transform a topographical substrate surface into a highly uniform, planar substrate surface.
In the competitive semiconductor industry, it is also highly desirable to have a high yield in CMP processes by producing a uniformly planar surface at a desired endpoint on a substrate as quickly as possible. For example, when a conductive layer on a substrate is under-planarized in the formation of contacts or interconnects, many of these components may not be electrically isolated from one another because undesirable portions of the conductive layer may remain on the substrate over a dielectric layer. Additionally, when a substrate is over-planarized, components below the desired endpoint may be damaged or completely destroyed. Thus, to provide a high yield of operable microelectronic devices, CMP processing should quickly remove material until the desired endpoint is reached.
The planarity of the finished substrate and the yield of CMP processing is a function of several factors, one of which is the rate at which material is removed from the substrate (the “polishing rate”). Although it is desirable to have a high polishing rate to reduce the duration of each planarizing cycle, the polishing rate should be uniform across the substrate to produce a uniformly planar surface. The polishing rate should also be consistent to accurately endpoint CMP processing at a desired elevation in the substrate. The polishing rate, therefore, should be controlled to provide accurate, reproducible results.
In certain applications, the polishing rate is a function of the relative velocity between the microelectronic substrate 12 and the polishing pad 41. For example, where the carrier 30 and the substrate 12 rotate relative to the polishing pad 41, the polishing rate may be higher toward the periphery of the substrate 12 than toward the center of the substrate 12 because the relative linear velocity between the rotating substrate 12 and the polishing pad 41 is higher toward the periphery of the substrate 12. Where other methods are used to generate relative motion between the substrate 12 and the planarizing medium 40, other portions of the substrate 12 may planarize at higher rates. In any case, spatial non-uniformity in the polishing rate can reduce the overall planarity of the substrate 12.
One conventional method for improving the uniformity of the polishing rate across the face of the substrate 12 is to vary the normal force (and therefore the frictional force) between the substrate 12 and the polishing pad 41 to account for the different relative velocities between the two. For example, in one conventional arrangement shown in FIG. 2, a carrier 30a can include a plurality of downward facing jets 35 (shown schematically in FIG. 2) that can direct high pressure air through a small cavity 39 and against the backside of the substrate 12, pressing the substrate 12 against the polishing pad 41. In one aspect of this arrangement, selected jets 35 can be closed or opened to vary the normal force applied to the substrate 12. For example, where it is desirable to reduce the normal force applied toward the periphery of the substrate 12 (relative to the normal force applied to the center of the substrate 12), selected jets 35 aligned with the periphery of the substrate 12 can be closed. One drawback with this approach is that it may be difficult and/or time consuming to change the number and/or location of the closed jets when the carrier 30a planarizes different types of substrates 12. A further drawback is that it may be difficult to accurately control the pressure applied by the jets because of the flow of gas from the jets 35 in the cavity 39 can be highly turbulent and unpredictable.
Another approach to varying the normal force applied to the substrate 12 is to use pressurized bladders, as shown in FIG. 3. For example, in one conventional approach, a carrier 30b can include a central bladder 36a aligned with the central portion of the substrate 12 and an annular peripheral bladder 36b aligned with the periphery of the substrate 12. The carrier 30b can also include an annular retaining ring 37 that is biased against the polishing pad 41 by an annular retainer bladder 36c. Each of the bladders 36a-36c is coupled with a corresponding conduit 38a-38c to a separately regulated pressure source. Accordingly, the pressure applied to the central bladder 36a can be increased relative to the pressure supplied to the peripheral bladder 36b to increase the normal force at the center of the substrate 12 and account for the lower relative velocity between the substrate 12 and the polishing pad 41 near the center of the substrate 12. One drawback with this approach is that it can be cumbersome to couple several different high pressure supply conduits to the rotating carrier 30b. Furthermore, it may be difficult to change the relative sizes of the bladders where it is desirable to change the relative sizes of portions of the substrate 12 subjected to different pressures.